Report No. 24/05
Task No. BE/200500003
Occurrence No. BO/200500620
Examination of failed thrust bearings from a Teledyne
Continental Motors GTSIO520M engine
Factual Information
1.1 Examination brief
The Australian Transport Safety Bureau (ATSB) was requested by
officers from the Civil Aviation Safety Authority (CASA) to conduct
an examination and analysis of several damaged thrust bearing
elements from the propeller drive shaft of a Teledyne Continental
Motors model GTSIO520M aircraft engine (serial number 810712).
Information received indicated that the bearings and accompanying
propeller shaft were removed from service during maintenance
activities; the discovered damage prompting the aircraft operator
to submit a defect report / service difficulty report (SDR) to
CASA. A second similar instance of bearing failure and shaft damage
was also reported, having occurred on a similar engine (serial
number 239168R) approximately one month previously.
The assessment of bearing compliance with the manufacturer's
specifications or the direct applicability of the particular
bearing part to the engine within which they were installed was not
within the scope of the investigation.
1.2 Items received
The ATSB received the full set of thrust bearing elements and
the propeller shaft from engine serial number (ESN) 810712 (figure
1). The propeller shaft from the earlier occurrence was also
provided (figure 2), however the associated thrust bearing set was
not available.
Figure 1. Propeller shaft and thrust bearings from
engine serial number 810712.
Figure 2. Propeller shaft from engine serial number
239168R.
As installed, the thrust bearings comprised two sets of
semicircular opposing plates, located against flange elements at
the rear of the propeller shaft (figure 3 ). The rear bearings,
carrying the primary propeller thrust loads, had sustained
extensive mechanical and thermal damage, whereas the forward
bearings showed little evidence of abnormal service and were
visibly sound (figures 4, 5). The undamaged (forward) bearing set
carried the rear surface identification '646260 G'. Damage to the
rear bearings prevented the recognition of any similar markings on
those items.
Figure 4. Contact (bearing) surfaces of thrust bearings
as-received.
Figure 5. Rear (backing) surfaces of thrust bearings
as-received.
The thrust bearings were understood to be of tri-metal
construction, comprising a surface layer of lead-tin babbit, over a
copper-lead intermediate layer on a steel backing. The SDR document
indicated the bearing set had operated for 433.4 hours since new
installation (TSN).
1.1 Examination findings
1.1.1 Rear thrust bearings
Close visual examination of the set of failed bearings
(attachments A - D) showed heavy scoring, gross disruption and
partial loss of the bearing alloy from the bearing running
(contact) surfaces (figure 6). Partial melting of the surface
alloys and other evidence of elevated temperatures was prevalent,
as was the blackening and discolouration of the surfaces where
alloy loss had occurred. The rear (back) faces of the bearings also
showed blackening and the accumulation of melted alloy (figure 7).
The lubrication channels on one bearing were partially filled with
transferred material that had been melted and dislodged from
adjacent areas.
Figures 6 and 7. Low-power microscopic view of a damaged
area on the contact and backing surfaces of a damaged thrust
bearing.
In isolated regions, the bearing alloy had completely separated
from the steel backing, with the affected areas characterised by
exposure of the comparatively flat, featureless interfacial
surfaces (figure 8).
Figure 8. Area on a damaged bearing showing complete
separation of the bearing alloy from the steel
backing.
Several metallographic sections taken transversely through one
bearing confirmed the basic tri-metal construction, with the
intermediate layer presenting as a coarse intermittent network of
lead within a copper alloy matrix. In numerous areas, the lead
network had interconnected, creating filled fissures (figure 9),
the larger of such broaching the external surface. Evidence of lead
migration to the steel backing interface was observed at and
adjacent to the areas of alloy separation, creating a continuous
lead boundary layer approximately 5-10 m thick.
Figure 9. Cross-sectional microstructure of a damage
bearing, showing the agglomeration of lead (dark phase) within the
copper bearing alloy and along the backing interface.
Unetched.
Scanning electron microscopy of the prepared sections confirmed
the metallographic observations, with back-scattered electron
imaging (figure 10) and x-ray dot mapping (figure 11) graphically
illustrating the lead agglomeration and migration to the backing
interface.
Figure 10. SEM image of the metallographic section,
illustrating the lead migration.
Figure 11. SEM X-ray map confirming the lead migration
(green phase) within the copper alloy (red phase) and at the steel
(blue phase) interface.
1.2.1 Forward thrust bearings
In contrast with the rear elements, the forward bearings
presented in an essentially undamaged condition (figure 12), with
very little evidence of metal-to-metal surface contact and no
evidence of thermal distress, overheating or physical degradation.
The rear surfaces were not fretted or rubbed to any significant
extent and showed no indication of improper seating, movement or
miss-installation.
Several metallographic sections, taken in a similar sense to
those from the rear bearings, presented a similar general
microstructure, with the intermediary alloy layer showing a
distinct as-cast (dendritic) distribution of lead within the copper
alloy matrix (figure 13). No evidence of fissuring or lead
migration to the backing interface was observed within the
cross-sections studied.
Typical thicknesses of the bearing component layers and backing
were established by measurement under the SEM, i.e.
Surface Pb-Sn babbit: 7 - 10 m (0.007 - 0.010 mm)
Intermediate Cu-Pb alloy: 715 - 725 m (0.715 - 0.725 mm)
Steel backing: 1,650 m (1.65 mm)
Figure 12. Undamaged running surfaces of the forward
bearing set.
Figure 13. Metallographic cross-section through an
undamaged bearing - no lead migration to the alloy
interface.
1.2.2 Propeller shafts
Both propeller shafts showed distinct discolouration and
evidence of localised heating in a band around the back face of the
rear thrust flange (figure 14). Similar discolouration was also
noted in a band around the rearmost ends of the reduction gear
teeth (figure 15), however the tooth contact surfaces themselves
showed no evidence of distress, uneven wear or excessive localised
friction.
The inside (bearing) surfaces of the rear shaft flanges (those
working against the failed bearing elements) showed heavy wear and
circumferential scoring around the contact path (figure 16). In
contrast, the opposite flange faces (figure 17) showed little if
any physical manifestation of service - abnormal or otherwise.
Figure 14. Discolouration of the rear thrust flange
surface typifying the localised overheating.
Figure 15. Discolouration (similar to figure 12) evident
on the ends of the reduction gear teeth.
Figure 16. Appearance and extent of scoring and wear
sustained by the propeller shaft rear thrust flange contact
surface.
Figure 17. Forward thrust flange (opposite that shown
above), presenting in sound condition.
Analysis
Damage to the propeller shaft thrust bearing assembly was
limited to the rear bearings and shaft flanges, those being the
components carrying the primary propeller thrust loads when under
power. The forward bearings and shaft flanges were undamaged and
showed no indication of anomalous service.
The rear propeller shaft thrust bearings from ESN 810712 had
failed as a result of gross localised overheating. The local
discolouration of the shaft flanges and adjacent surfaces and the
partial melting and microstructural changes within the bearing
alloy attested to the excursion in temperatures to a level well
above the normal component operating range. The physical loss of
sections of bearing alloy from the backing material was a direct
manifestation of the overheating, with the elevated temperatures
causing the lower melting point lead alloy to agglomerate and
migrate to the steel interface, where it weakened the normal bond
and allowed the break-up and separation of the bearing material.
There was no evidence of a deficiency within the construction or
make-up of the bearings examined, nor was there any evidence that
the bearings had been improperly installed.
In a general sense, the overheating of bearings results from the
generation of frictional heating at a rate greater than the
assembly and environment is able to conduct it away. Heating, from
surface and lubricant frictional effects, is a function of numerous
interrelated factors including:
- bearing operating (transmitted) loads
- clearances
- lubricant properties
- lubricant quantities and flow rates
- relative surface speeds
- surface conditions and finishes
- bearing materials
The investigation was not able to directly identify which of the
identified factors were contributory to the failures sustained,
however it is suggested that issues relating to the initial bearing
clearances, lubricant and operating (thrust) loads would be the
most likely in terms of the general nature and function of the
assembly.
Conclusions
The following conclusions, in terms of the bearing failures,
were drawn from the examination of the supplied components:
- The bearings from ESN 810712 had failed as a result of gross,
localised frictional overheating, resulting in the physical and
microstructural degradation of the bearing alloy. - There was no evidence that a manufacturing defect, material
anomaly or other deficiency within the bearing components
themselves had contributed to the failure. - There was no evidence found to suggest that the bearings had
been improperly installed. - The investigation was not able to directly identify the
proximate cause/s of bearing failure, however it is suggested that
initial bearing clearances, lubrication and loading were most
likely in terms of the nature of the failure and the general
function of the assembly.